This invention relates to birefringence measurement systems, examples of which are described in U.S. Pat. No. 6,985,227, hereby incorporated by reference.
A schematic diagram of an optical setup 20 of one embodiment of such a system is shown in FIG. 1. That system employs as a light source 22 a polarized, 1550-nm diode laser. The light beam B1 from the source 22 passes through a polarizer 24 that is oriented at 45 degrees. The system also includes a photoelastic modulator (PEM) 26 oriented at 0 degrees and operated at 42 KHz. The source 22, polarizer 24, and PEM 26 components of the optical setup are typically secured in a casing or cabinet and are collectively referred to as a source module 28.
A sample holder 30, which can be mounted on a computer-controlled X-Y stage, is located in the path of the optic axis 25 of the optical setup 20, thereby to allow the beam to scan various locations of an optical element or sample 32. The light passing through the sample is then detected for determination of the birefringence of the sample.
The light beam B2 that emanates from the sample (hereafter referred to as the “sampled beam” B2) is directed through another PEM 34 that is oriented at 45 degrees and operated at 47 KHz. After passing through an analyzer 36 oriented at 0 degrees, the sampled beam B2 is directed into the receiving or active area 38 of a Ge-photodiode detector 40. The PEM 34, analyzer 26 and detector 40 components of the optical setup are collectively referred to as a detector module 42 that includes an entrance aperture 44 through which the sampled beam B2 enters the detector module.
The light source beam B1 (in this case, laser light) is well collimated, and compact detectors (that is, having relatively small active areas 38 on the order of 1-6 mm) may be employed, especially where the sample 32 has smooth, parallel surfaces normal to the incident light beam B1, or exhibits other physical characteristics such that the beam path is substantially unaltered in passing through the sample.
The cost effectiveness and efficiency of a birefringence measurement system as just described is enhanced when the system can analyze a wide variety of sample types and sizes. Moreover, it is important to maintain the mechanical reliability and repeatability of measurements for a given system by minimizing or eliminating the need for repositioning the components of the source module 28 and detector module 42 irrespective of changes in the size or shape of the sample.
Some samples for which birefringence measure is desired are shaped or configured so that the sampled beam is spread as a result of diffusion, defocusing, scattering, fanning or other mechanisms. For instance, as diagrammed in FIG. 2, a sample 232 may be formed of material, such as polycrystalline silicon, which will alter the source light beam B1 so that the sampled beam B2 emanating from the sample 232 is spread, as compared to the incident beam B1, such that only a small portion of the light that passes through the sample and then through the entrance aperture 44 will impinge upon the active area 38 of the detector 40. Thus, the intensity of the light reaching the detector 40 is too low for accurate detection that would permit determination of birefringence characteristics of the sample. This effect becomes more problematic when even larger samples are tested. The foregoing problem with beam spreading associated with samples such as polycrystalline silicon or the like will also arise with other types of sample material. For example, single crystal silicon or other material may have a surface roughness that can similarly diffuse or spread the light beam directed through it.
This invention is directed to a method of addressing the foregoing problem attributable to such beam-spreading samples without reconfiguring the optical setup of the birefringence measurement system (by moving the detector, changing the light source power, etc.) in a manner that would sacrifice the cost effectiveness, efficiency, mechanical reliability and repeatability of measurements for such systems. The system of the present invention is thus reliably usable with a wide range of samples, regardless of the size, shape, finish, or other physical characteristics of the material.